1. Field of the Invention
This invention relates to a device and a method for preheating a die used in the hot extrusion of aluminum alloys and the like, and more particularly relates to a device and a method for rapidly heating such a die.
2. Description of the Prior Art
It is common in arts of extruding metals, such as aluminum alloys, into desired shapes, to heat a billet of an aluminum alloy to a very high temperature and to also preheat the die which defines the configuration of a resulting extrusion. However, when the die alone is preheated it becomes difficult and time consuming to handle the die and to attach it to an extruder. If too much handling time is required the die cools to room temperature.
To avoid such problem, there have been attempts and proposals to have the die built in an outer ring or sheath, so that the outer ring and die may be heated in an atomosphere-heat-treating furnace. In such preheating furnaces, it is a common practice to use a burning type heating furnace or a resistance-heating furnace for heating the die to a temperature from 450.degree. C to 500.degree. C.
However, because of the nature of an atomosphere-heating furnace, it takes three to five hours for the furnace to reach the desired temperature of 450.degree. to 500.degree. C and another 3 to 5 hours for the die to reach that temperature. For this reason, extra dies should be placed in the preheating furnace so they will be readily available if an unexpected need arises. Unfortunately, this shortens the service life of a die due to the vigorous oxidation thereof, with the result that many man hours must be spent for repairing dies. Furthermore, inasmuch as the furnace should be heated continuously to maintain it at the desired temperature, a great amount of electric power is used, thereby increasing operating costs.
There have been attempts to overcome the aforementioned disadvantages by using an induction-heating process to heat dies. One example of the aforesaid induction-heating type furnace is shown in FIG. 1, wherein a cylindrical die 1 having a nozzle in its center and an outer ring or sheath 2, to provide a die assembly 3, is placed on one end of a leg 4a of a fixed yoke, having a `C` shape, while a coil 7 is wound around another leg 4b of the yoke and connected to an electric power source 5. A power-factor improving capacitor 6 is connected across the electric power source 5. A movable yoke 8 is secured by hinge means 9 to one end of the yoke 4, so that the die may be heated by a magnetic flux passing through the die 1. However, such an attempt requires a relatively long time for heating and poses a shortcoming of damaging the surface of the die 1 due to magnetic oscillation.
Another prior art heating device and method which overcomes the disadvantages of the previous method, is shown in FIG. 2. There, the die 1, having an extrusion nozzle 1a, is fitted in an intermediate insulating ring 10 which in turn is fitted in an outer ring 11 to provide a die assembly. The die assembly 3 is fitted in a bobbin or tube 13 in concentric relation thereto, while an induction-heating coil 7 is wound around the outer periphery of the bobbin 13. A power-factor improving capacitor 6 is connected across an electric power source 5, to which the aforesaid coil 7 is connected. Thus, magnetic flux is produced in the axial direction of the bobbin, thereby induction-heating the die.
The aforesaid device and method permits rapid heating of the die to a temperature of about 500.degree. C within a duration as short as 10 minutes. In addition, the radiation of heat from the die 1 is prevented by means of the insulating intermediate ring 10. However, for reasons which will be explained below this method is only partially successful in achieving uniform temperature-distribution in heating the die.
More particularly, referring to FIG. 2, the maximum temperature difference between the center portion and outermost periphery of the die is given as follows: ##EQU1## wherein r: radius of die;
.theta..sub.s o/c: temperature at the outermost periphery of die; PA1 .theta..sub.c o/o: temperature in the central portion of die; PA1 kc: thermal conductivity depending on the type of material of die; PA1 po: power density; and PA1 F(2r/.delta.): a function of the diameter 2r of the die and a permeable depth .delta. and is in the range of 0.6 to 0.9. The value .delta. is an electric constant depending on the type of material of the die and the frequency of the electric heating source.
As is clear from the aforesaid formula (1), the larger the diameter of the die, the greater will be a temperature difference .vertline..theta..sub.s - .theta..sub.c .vertline., with increased possibility of cracking in the die. For avoiding the above shortcoming, the power density Po is limited to a lower value, or an input to the electric heating source is turned on and off repeatedly for uniformly heating the die, while the temperatures at the center and outermost periphery of the die are continuously measured. This leads to the result that the die is no longer rapidly heated. The measured values of the temperature distrubution of a die according to the aforesaid method and device were: EQU .theta..sub.s = 550.degree. C, .theta..sub.c = 250.degree. C, and .vertline..theta..sub.s - .theta..sub.c .vertline..sub.max = 300.degree. C,
for a case where the die diameter was 200 mm, the die thickness was 40 mm, the induction coil input was 78 kw, and the heating duration lasted for 5.3 minutes.